The invention relates to projection televisions and projection displays, and more particularly to compact rear-projection displays.
Rear screen projection television displays have found great consumer interest due to a large image size. A primary disadvantage is the set volume and depth. There is great desire to reduce set size but this is difficult as a practical matter, and is costly with present CRT based projection systems.
Microdisplay based systems such as Liquid Crystal Display (LCD), Digital Micromirror Device (DMD), or Liquid Crystal on Silicon (LCoS) can achieve much smaller system volume than for CRT projection displays. This is due to a smaller engine size and projection from a single projection lens as opposed to three tubes and lenses for CRT displays. Very attractive system dimensions have been announced for these microdisplay systems. Recent product announcements include for example a 55-inch diagonal wide screen set with a system depth of 18 inches, and a 50-inch diagonal wide screen set with a depth of 35 cm. These sets make use of a single large mirror, known in the art as a fold mirror.
Narrow depth is obtained with a wide-angle projection lens. To achieve this, light from the projection lens must be directed to the fold mirror which is placed upwards and to the rear of the unit. One way to achieve this desired pointing direction by incorporating a fold mirror into the projection lens, making the projection lens into a so-called folded projection lens. Examples of compact rear projection systems that have been constructed with the use of a folded projection lens include the Sony GRAND WEGA and the Sharp LC-R60HDU. Both these systems use three transmissive LCD panels. For these systems, the folded projection lens must be used in order to align the engine parallel to the bottom of the cabinet. However, addition of a fold mirror leads to increased lens cost, reduced performance, or both.
It is advantageous therefore to reduce the depth of rear-projection displays, while keeping the manufacturing costs as low as possible. To address one or more of the above issues, in one aspect of the invention, the applicant proposes utilizing a prism, for example a polarizing beam-splitter (PBS) present in an LCoS projection display, instead of conventional structure for a folded projection lens.
Accordingly, in a first aspect of the invention, a rear-projection display system comprises a light engine, a beam-splitter that receives light from the light engine, a projection lens that receives an image from the beam-splitter, a rear-projection screen, and a mirror that receives the image from the projection lens and reflects it onto the screen. The beam-splitter has a reflecting element, receives an image, and reflects the image off the reflecting element at an angle of incidence that is substantially unequal to 45 degrees.
In a second aspect of the invention, a rear-projection display system comprises a light engine; beam splitting means that receives light from the light engine, receives an image, and reflects the image with an angle of incidence substantially unequal to 45 degrees; a projection lens positioned to receive the image from the beam splitting means; a mirror positioned to receive the image from the projection lens; and a rear-projection screen positioned to receive the image from the mirror.
In a third aspect of the invention, a rear-projection display method comprises using light from a light engine to form an image; reflecting the image off a reflecting element to a projection lens; projecting the image to a mirror; and reflecting the image from the mirror to a rear-projection screen. The image is reflected from the reflecting element at an angle of incidence that is substantially unequal to 45 degrees.